[luoguP1186] 玛丽卡（spfa）

 1 #include <queue>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 
 6 using namespace std;
 7 
 8 const int MAXN = 1001;
 9 int n, m, cnt, ans, qu, qv;
10 int head[MAXN], to[MAXN * MAXN], next[MAXN * MAXN], val[MAXN * MAXN], dis[MAXN], pre[MAXN];
11 bool vis[MAXN], flag;
12 queue <int> q;
13 
14 inline void add(int x, int y, int z)
15 {
16     to[cnt] = y;
17     val[cnt] = z;
18     next[cnt] = head[x];
19     head[x] = cnt++;
20 }
21 
22 inline void spfa(int u)
23 {
24     int i, v;
25     while(!q.empty()) q.pop();
26     memset(vis, 0, sizeof(vis));
27     memset(dis, 127 / 3, sizeof(dis));
28     q.push(u);
29     dis[u] = 0;
30     while(!q.empty())
31     {
32         u = q.front();
33         q.pop();
34         vis[u] = 0;
35         for(i = head[u]; i != -1; i = next[i])
36         {
37             v = to[i];
38             if((u == qu && v == qv) || (u == qv && v == qu)) continue;
39             if(dis[v] > dis[u] + val[i])
40             {
41                 dis[v] = dis[u] + val[i];
42                 if(!flag) pre[v] = u;
43                 if(!vis[v])
44                 {
45                     vis[v] = 1;
46                     q.push(v);
47                 }
48             }
49         }
50     }
51 }
52 
53 int main()
54 {
55     int i, x, y, z;
56     scanf("%d %d", &n, &m);
57     memset(pre, -1, sizeof(pre));
58     memset(head, -1, sizeof(head));
59     for(i = 1; i <= m; i++)
60     {
61         scanf("%d %d %d", &x, &y, &z);
62         add(x, y, z);
63         add(y, x, z);
64     }
65     spfa(1);
66     flag = 1;
67     for(i = n; i != -1; i = pre[i])
68     {
69         qu = i;
70         qv = pre[i];
71         spfa(1);
72         if(dis[n] != 707406378) ans = max(ans, dis[n]);
73     }
74     printf("%d", ans);
75     return 0;
76 }